It is known that excessive transformer inrush current would be caused whenever a transformer is going to be energized by the power system source-side breaker closing, and various bad effects such as temporary voltage drop, three-phase current unbalance as well as severe current waveform distortion would be caused on the adjacent power loads.
It is also known that the transformer inrush current is caused as a phenomena closely related with the transformer residual fluxes that appear in the transformer three-phase steel cores whenever it is tripped by the related breaker, and the initial energizing fluxes caused when the transformer is again energized in the next chance. So, quite accurate estimation or calculation of the caused residual fluxes in the transformer is essential in order to effectively restrain the energizing inrush current.
In this respect, there has been made often an easy explanation about the residual flux of a three-phase transformer, that is for example, residual fluxes appears whenever a three phase transformer under no load condition is cutoff from the power source-side by a connected breaker tripping, and the magnitudes of the residual fluxes appearing in the three phase transformer cores are the fluxes values just at the instant timing of the transformer cutoff by the related breaker tripping operation. By the way, no-load condition of the transformer means the condition where the high voltage side breaker is closed and the low voltage side (load side) breaker is opened so that power is not supplied to the load through this transformer.
Or for example, a conventional transformer inrush current control equipment is provided with a voltage memorizing unit to memorize the three phase voltage values just at the instant of the breaker tripping, and when the transformer is energized by the breaker closing, the breaker closing timing is controlled so that the three-phase voltage phase angles at the instant of the breaker closing become coincident with that of the memorized three phase voltage angular timing (see for example Japanese Patent Provisional Publication No. S55-100034).
Another example of conventional transformer inrush current restraining control equipment is provided with a voltage measuring unit to measure three-phase voltages of the transformer terminals and that of power source-side, the transformer core residual fluxes calculating unit which calculate the fluxes by the transformer three-phase voltages integration and to memorize the fluxes values and the polarities just at the angular timing of the breaker tripping as so to speak residual fluxes, and further the breaker closing phase angular timing controlling unit to restrain the inrush current.
The breaker closing phase angular timing controlling unit control the breaker closing angular timing so as to meet within the time interval where the polarities of the three phase initial energized fluxes energized by a system source-side voltages at the breaker closing timing and the polarities of the memorized residual fluxes become coincident in each phase (see for example Japanese Patent Provisional Publication No. 2008-140580).